1. Field of the Invention
This invention relates to a computer system including a central processing unit (CPU) memory system to retain data and to conserve electrical power by utilizing volatile solid state memory devices, such as DRAM (Dynamic Random Access Memory) semiconductor memory chips capable of retaining data, in a low power, self refresh mode whenever the memory system is not being accessed by the CPU.
In addition, the invention relates to a CPU and a memory system wherein the retention of data within a volatile solid state memory device is maintained in a self refresh mode by an internal power supply during times when an external power supply voltage falls below a predetermined level.
2. Prior Art
Current computer systems consume relatively large amounts of electrical energy to power the components comprising the computer system, thereby making demands on batteries used in portable computer systems. Sources of this energy consumption are the microprocessor computer chip, the volatile memory modules that retain information as long as a voltage is applied to them, and usually a hard disk drive and CD-ROM drive. The increase in computer chip speeds and increase memory module sizes each also increase demand for electrical energy. Hard disk drives and CD drives which continuously spin whenever the computer system is "powered on" are a substantial and constant source of battery energy. Successive generations of portable, laptop or notebook computers contain computer chips which perform at higher operating speeds, or contain more memory, or larger hard disk drive capacity. Each of these additions to the computer system increases the demand on the electrical power source and shorten the use of battery operation.
The batteries contain a limited amount of electrical energy to supply the electrical demands of the portable, laptop or notebook computer system. Larger batteries contain the capacity to provide electrical energy for a longer time than smaller batteries. However, as battery size increases to compensate for the increased electrical demands the weight of the portable, laptop or notebook computer also increases. Computer designers must continually trade off electrical power capacity against overall computer system weight.
Power Management methods and tools are designed in current portable, laptop and notebook computers to conserve battery power. Such methods and tools include, operating the computer chips on lower voltage than the standard 5 volt logic (operation on a reduced voltage also reduces the heat generated in portable, laptop or notebook computer systems), placing the computer chip in suspended or deep sleep mode when the computer chip detects no activity for a predetermined period of time, and powering down the hard disk drive after a user defined time period.
These methods and tools, while increasing battery life also have disadvantages. The methods and tools are essentially time based. Under such a time based system access to the peripheral devices, such as hard disk drives, could be denied as the peripheral device could itself be in a power down, or power savings mode. Further, setting the disk drive power down time period to too small a value can actually decrease battery life as the need to access the disk drive requires the disk drive be returned to a fully spinning condition. This return to a fully spinning condition can sometimes require more power than would be required to maintain the disk drive in the full spin operational mode.
Computer systems contain different forms of semiconductor memory used to store programs and data. The most common of these memories are Programmable Read-Only Memory (PROM), EPROM (Erasable PROM), EEPROM (Electrically Erasable PROM), FLASH and volatile solid state memory devices which maintain information only when a predetermined voltage is applied to them typically used for Random Access Memory (RAM). PROM memory is a memory where the contents, stored as a series of logical highs and lows(1 and 0) are permanently stored and unchangeable. When power is removed from the computer system, PROM memory retain the series of logical highs and lows stored within it. Similarly, EPROMs and EEPROMs, permanently retain the data stored with their memory, but have the capability to erase the logical highs and lows and replace the memory contents with new and different data patterns. Typically the EPROMs use Ultra-violet light as an eraser means, and EEPROMs uses electrical means as the eraser.
FLASH memory is similar to EEPROM memory. In each of these memory types, the storage of the data or programs in these memories is permanent, whether power is applied to the computer system or not. Such memory is referred to as non-volatile memory. As an example, Computer BIOS(Basic Input/Output Sequence) programs are stored in some form of PROM; where the older computer systems had BIOS program stored in EPROM, more current computer systems retain the BIOS in FLASH to allow for reprogrammability. When power is applied, the program stored in the BIOS begins the sequence to load the full operational program (such as Windows 3.11 or Windows95/NT) into the computer memory. PROM memory is used when size and power consumption are not critical factors, the program instructions do not change often and the program instructions are required to remain independent of the input power state.
Unlike PROM, Random Access Memory, for example DRAM or any solid state volatile memory device that maintains information when a voltage is applied to it, is used in a computer system when the program instruction changes often and the need for data retention is generally for a short period of time. Application programs, such as WORDPERFECT (TRADEMARK), Microsoft Word (TRADEMARK), and Lotus 123 (TRADEMARK) are programs loaded into computer RAM when they are to be executed and remain in RAM through the period of execution. Examples commonly used RAM memory include devices Static (SRAM) and Dynamic (DRAM). Both SRAM and DRAM retain the contents of their memory states when electrical power is provided and maintained on the devices. Whenever electrical power is removed from the devices, the memory contents of the device is lost and irretrievable. Memory systems that lose their data when electrical power is removed are called "volatile memories" in the art. When electrical power is intentionally removed, such as in an orderly shutdown of the computer system, there is no intent to retain RAM contents. But when the electrical power is unintentionally and unexpectedly removed, such as in cases of power outages, the lose of the contents of the RAM could be devastating.
To protect the contents of the RAM memory under such unexpected conditions, it has become a common practice to use an external Uninterrupted Power Supply(UPS) to maintain electrical power to the computer system during the such unexpected power outages. The UPS is positioned between the source of the input electrical power and the computer system. The UPS provides sufficient power to enable a user to save the contents of a program in operation and complete the current application program. When main electrical power is restored, the user can restart the application program and begin program operation again. Without the protection of UPS, the data entered and application program in RAM would be irretrievably lost.
One embodiment of the present invention is a memory system including solid state volatile memory devices capable of maintaining information when a voltage is applied to the devices, for example DRAM or the like. The invention prevents the loss of data due to unexpected power outages and also prevents errant control and address signals to the memory devices by monitoring the input electrical power source to the memory devices for acceptable conditions, and electrically isolating the memory devices from signals received on the control lines and address lines and switching to an alternate internal electrical power source, typically a battery, whenever the input power source is unacceptable.
The memory system maintains the integrity of the data retained by the memory devices by isolating the devices from the external power source, control lines and address lines and placing the memory devices into a power down self-refresh mode which will maintain the data using a minimum of electrical power.
The prior art U.S. Pat. No. 5,798,961 discloses a SIMM non-volatile memory module having a volatile memory array, a battery circuit, a charging circuit and a control circuit. The control circuit in connection with the battery circuit monitors the external power supply from the SIMM_VCC line. If the external power source falls below a predetermined voltage then the battery circuit maintains the integrity of the data by supplying power to the volatile memory array. The data retained by the memory module is susceptible to errant control signals and address signals that may appear on the control and address lines during times of unstable power in the system.